What is a Thread of Execution? (C#)

So I was going through Microsoft Virtual Academy C# Fundamental by Bob Tabor and during a video, he said something about the thread of execution leaving the scope and reducing the reference count. What is a "thread of execution" and how can it leave the scope? I also don't understand how that reduces the reference count of an object, I mean "thread of execution" doesn't sound like something related to an instance of a class to me. The video is here for further information: https://mva.microsoft.com/en-US/training-courses/c-fundamentals-for-absolute-beginners-16169?l=ymM4awQIC_5206218949